A device for coupling railroad cars to each other has a structure with a coupling pin 4 which joins a coupler 1 to a shock absorber 2 which absorbs and buffers shocks generated mutually between the cars while accelerating and decelerating as they travel, via a coupling joint 3 provided with insertion holes 3a and 3b in a cross-wise direction (Patent Reference 1, FIG. 7 and FIG. 8). This coupling device is able to incline in the direction of width of the cars and in the direction of height of the cars. When the coupler 1 and the shock absorber 2 form a set, it is referred to as a “coupling device.”
A coupling device which couples railroad cars to each other, such as is disclosed in Patent Reference 2, for example, is employed as a coupling device which couples tilting railroad cars to each other, to make possible high-speed travel on a curved rail.
As shown in FIG. 5, the coupling device disclosed in Patent Reference 2 has a rounded concave surface 12a forming a front inner surface of a shock absorber frame 12 which holds a shock absorber 11, and a cylindrical surface 12b with a vertical inner surface which extends to the rounded concave surface 12a. A coupling joint 13, which is intended to engage with the rounded concave surface 12a of the shock absorber frame 12, is provided with an insertion hole 13b in the shape of a short cylinder, into which is inserted a coupling pin 14 in a direction perpendicular to the axis, and has an outer peripheral surface formed in a rounded convex surface 13a. 
An assembly of the above described coupling device operates in the following manner.
First, the coupling joint 13 is inserted from the open side 12c of the shock absorber frame 12, pushing in a forward direction for the rounded concave surface 12a to accommodate the rounded convex surface 13a. 
Next, the shock absorber 11 with front and back held by a follower plate 15 is inserted into the shock absorber frame 12, and a rear portion 16a of a coupler 16 is inserted into the short cylinder of the coupling joint 13 from the front of the shock absorber frame 12.
Finally, the coupling pin 14 is inserted into the insertion hole 13b, and the rear portion 16a of the coupler 16 couples with the shock absorber frame 12, after which a locking screw 17 anchors a stop plate 18 in a setting groove 13c provided at both ends of the insertion hole 13b, so that the coupling 14 will not come lose. In this state, a rounded concave surface 15a provided to the front surface of the follower place 15 contacts a rounded convex surface 16aa of the rear portion 16a of the coupler 16.
This enables a smooth rotation and a vertical and width-wise inclination of the coupler 16 with respect to the shock absorber frame 12.
In areas where high-speed railroad cars and conventional railroad cars travel, there are instances when the high-speed railroad cars and the conventional railroad cars are coupled along their routes, and instances when the high-speed railroad cars and the conventional railroad cars are decoupled to run separately along their routes. In such cases, the front cars of the high-speed train must be coupled to the conventional railroad cars.
However, there are many cases in which the front part of the front car for a high-speed train, which has special vehicle components in the front part, has no extra space at all on the right and left sides, or hardly any space on the top of a position where a coupling device is mounted. In such cases, after installing a coupling device used for coupling tilting cars to the front part of the front cars of the high-speed train, there is no space to remove the entire coupling device during maintenance, and to mount it after maintenance.
Therefore, when the coupling device disclosed in Patent Reference 2 is installed at the front part of the front car of a high-speed train, the locking screw for anchoring the stop plate in position on the right and left side of the coupling device during maintenance is removed, making it impossible to disassemble and remove the coupler and the shock absorber. Even if the coupling device is rotated 90° from its installed position during maintenance, there is hardly sufficient space above, so the operation of removing the locking screw is very difficult.